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We have a middleware for that! You can look at http://github.com/iron/persistent, which does wrap an RWLock in an Arc. It also provides a struct to make your middleware Share, so you can do similar things.

We also have session, which works on a HashMap under an Arc, although it isolates the mutable data to a session, instead of sharing it across isolated requests.

I would guess `Arc<Mutex<T>>`, `Arc<RWLock<T>>` and `Arc<SomeConcurrentContainer>` would all work (cloning them is a ref-count - the handle gets copied, not the contents).

You are looking for https://github.com/iron/persistent, which offers two utility middleware for exactly this.

Very nice, thanks.